REUTERS - Two men were jailed for life on Thursday for the murder of two French students last year, as the Conservatives demanded to know how one of the defendants had mistakenly been let out of prison shortly before the killings.

Dano Sonnex, 23, was given a minimum term of 40 years and Nigel Farmer, 33, was told he must serve at least 35 years.

They had been found guilty at the Old Bailey of stabbing the 23-year-old French postgraduates a total of 244 times during a three-hour torture ordeal at a south London flat last June.

Laurent Bonomo and Gabriel Ferez died in what the prosecution called a frenzied attack of "unmitigated evil".

But it emerged during the trial that Sonnex, who had been jailed for a previous knife attack and armed robbery, had been mistakenly let out on licence despite taking drugs and setting fire to his cell.

Even when the error was spotted, it was two weeks before police took Sonnex back into custody. By then the students were already dead.

Justice Secretary Jack Straw has apologised to the victims' families for the blunder, while David Scott, head of the probation service, has resigned.

The case has echoes of that of city financier John Monckton who was stabbed to death in his Chelsea home in 2004 by a man who had just been released halfway through a 12-year sentence for attempted murder and who been assessed as being at low risk of re-offending.

Sonnex, of Peckham, south London and Farmer, of no fixed address, had burst into Bonomo's bedsit in New Cross and subjected the two students to an "orgy of bloodletting" before setting the flat ablaze.

They stole games consoles, mobile phones and bank cards which they used to withdraw 360 pounds.

Fuelled by drink and drugs, the robbers egged each other on to an "inhuman attack of brutal and sustained ferocity", the jury heard.

Hours later, Farmer torched the bedsit, leaving firefighters to find both victims bound at the ankles and wrists and their heads wrapped in towels. Bonomo's fold-out bed was found saturated in blood.

Sonnex and Farmer had blamed each other at the trial but prosecutor Crispin Aylett described the crime as a "joint enterprise of unmitigated evil".

Conservative Shadow Justice Secretary Dominic Grieve said the case had revealed serious failings across the criminal justice system.

"The Justice Secretary has had to accept full responsibility, because these terrible events are a direct result of serial ministerial mismanagement," he said.

"Jack Straw must come to parliament at the earliest opportunity to explain the full facts of how such chronic failings could have occurred -- just three years after the government claimed it had learnt the lessons from the probation failings that led to the murder of John Monckton."